A method of the kind referred to above is, for example, disclosed in U.S. Pat. No. 4 835,718.
The known control method provides that safety-reference positions in the form of points are defined before the start of the actual measuring program. These safety-reference positions lie on the path pregiven for the probe head or contact ball. In the case of a disturbance, the probe can be moved back to these points or along these points. These safety-reference positions, that is, the coordinates of the points concerned must be made part of the control data when programming an automatic measuring sequence. For this reason, the method is relatively complex to program and corresponding preparatory time is needed to prepare the automatic sequence. Furthermore, little flexibility is provided because the safety-reference positions or their point coordinates are fixed for the particular workpiece.
However, there is often the desire to not measure workpieces completely; instead, only specific features or geometric elements on the workpiece are measured either because the other features are not sufficiently important for the measuring task concerned or measuring time is to be reduced or both. For such a feature-referred measurement only specific portions of the CNC sequence are selectively processed and for this measurement the known method is not well suited. This is so because in the course of a measurement referred to specific features, a control module automatically generates the travel path between the geometric elements which are to be contacted specifically for the measuring task according to pregiven criteria. Fixed pregiven safety-reference positions on the workpiece are therefore only poorly compatible with this control method.